Conventionally, alumina (Al.sub.2 O.sub.3) is used as a dielectric material for microelectronic packages. It has excellent electrical (insulating), thermal and mechanical (especially strength) properties. Alumina based packages generally containing 4-10 wt. % glass require sintering temperatures above 1500.degree. C., which necessitates the use of refractory metals such as molybdenum or tungsten for the electrical interconnections so that the metal can be co-fired with the package. These metals have poor electrical conductivity as compared to highly conductive metals such as copper, and secondly, they require the use of strongly reducing atmospheres during co-firing, necessitating expensive furnace systems.
The development of multilayer ceramic circuit boards is toward higher frequency, higher density and higher speed devices. Al.sub.2 O.sub.3 has a relatively high dielectric constant of about 9.9, causing high signal propagation delay and low signal-to-noise ratio (crosstalk). The signal propagation delay (t) in ceramic substrates is affected by the effective dielectric constant of the substrate (k') according to the following equation: EQU t=(k').sup.0.5 /C
where C is the speed of light. It can be found that the signal propagation delay can be dramatically reduced by a reduction in the effective dielectric constant of the substrate. For example, if the dielectric constant of a material is reduced from 10 (approximately the k' of Al.sub.2 O.sub.3) to 5, the signal propagation delay can be reduced by 30%. A small signal delay is especially important for the substrate housing a chip with a very dense integrated circuit, for instance, very high speed integrated circuit.
Furthermore, alumina has a coefficient of thermal expansion of about 7.4.times.10.sup.-6 /.degree.C. (in the 20.degree.-200.degree. C. range) as compared to 3.4.times.10.sup.-6 /.degree.C. for silicon. This mismatch in thermal expansion results in design constraints and reliability concerns when attaching a silicon wafer to the substrate.
Heretofore, most of the dielectric materials used in multilayer circuits have been conventional thick film compositions. A typical circuit is constructed by sequentially printing, drying and firing functional thick film layers atop a ceramic substrate which is usually 92-96 wt. % Al.sub.2 O.sub.3. The multiple steps required make this technology process intensive with the large number of process steps and yield losses contributing to high costs. Thick film technology nevertheless fills an important need in microelectronics and will continue to do so in the foreseeable future.
Recently, dielectric thick film compositions with low dielectric constants have been introduced. However, ceramic substrates with low dielectric constants less than 4.5 and thermal expansion coefficients equal to that of silicon (3.4 ppm/.degree.C.) are not readily available.
Low temperature co fired (LTCF) technology has been recently introduced as a method for fabricating multilayer circuits. This technology offers the combination of the processing advantages of HTCF technology and the materials advantages of thick film technology. These LTCF tape systems have firing temperatures below 1000.degree. C. and allow the use of high conductivity metals such as silver, gold, silver/palladium and copper (copper, however, requires reducing atmospheres). Most of these tape systems have dielectric constants between 6 and 8 and encompass a range of thermal coefficient of expansion (TCE).
Currently, we are not aware of any readily available low temperature co-fired dielectric tape system using a glass plus ceramic approach that offers both low dielectric constant (less than 4.5) and a TCE matched to silicon (3.4 ppm/.degree.C.).
A method for producing a multilayer ceramic circuit board for use with copper conductors is described in U.S. Pat. No. 4,642,148 issued to Kurihara et al. Ceramic compositions comprising 10-75 wt. % alpha-alumina, 5-70 wt. % non-crystalline quartz (fused silica) and 20-60 wt. % borosilicate glass are disclosed. The dielectric constants of the fired materials ranged from 4.8 to 9.6.
U.S. Pat. No. 4,672,152 issued to Shinohara et al describes a multilayer ceramic circuit board in which the ceramic is prepared from a mixture of 50-95 wt. % crystallizable glass and 5-50 wt. % ceramic filler. The material has a dielectric constant between 5.1 and 6.0 and a flexural strength above 150 MPa. The crystallizable glass consists of 5-20 wt. % lithium oxide, 60-90 wt. % silicon dioxide, 1-10 wt. % aluminum oxide and 1-5 wt. % alkaline metal oxide other than lithium oxide. The ceramic filler is selected from the group of silicon dioxide, .beta.-eucryptite (LiAlSiO.sub.4) and aluminum oxide.
U.S. Pat. No. 4,755,490 issued to DiLazzaro describes a low firing temperature ceramic materials having dielectric constants between 4.5 and 6.1. The materials had coefficient of thermal expansion between 3.9 and 4.2 ppm/.degree.C..times.10.sup.-6. Example 11 shows k'=4.5 and TCE=3.9. The material is formed from a mixture of 10-50 wt. % alumina, 0-30 wt. % fused silica and 50-60 wt. % (approximately 60-70 vol. %) of a frit composed of about 4 wt. % CaO, about 12 wt. % MgO, about 29 wt. % B.sub.2 O.sub.3, and about 42 wt. % SiO.sub.2. The compositions are fired at a temperature below 1000.degree. C.
U.S. Pat. No. 4,788,046 issued to Barringer et al describes a glass-ceramic packages for integrated circuits having low sintering temperature. The sintered compositions are formed by coating ceramic particles with glass, separating the coated particles from the glass and then forming the coated particles into a green compact. The material with the lowest dielectric constant (4.5) is obtained using quartz. This material has a thermal expansion coefficient greater than 5.5.
U.S. Pat. No. 4,879,261 issued to Burn describes a low dielectric material having a dielectric less than 5.0. The material is formed from a mixture of finely divided particles consisting essentially of 70-85 wt. % silica and 15-30 wt. % zinc borax flux which is fired to 1065.degree. C. in an oxidizing atmosphere. The composition can be used to make green tape and multilayer devices having internal copper conductors such as multilayer capacitors and multilayer interconnects.
From the foregoing, it can be seen that there is a substantial need for a low temperature co-fireable tape dielectric which (1) has a low dielectric constant (less than 4.5), (2) has a thermal expansion coefficient very close to the value for silicon (3.4 ppm/.degree.C.), and (3) can be fired in air at a low temperature (less than 950.degree. C.), thus permitting the use of high conductivity metallurgies such as gold, silver and silver/palladium.
The principal object of the invention is to provide a material that can be sintered into a body that has a dielectric constant of less than 4.2, a thermal expansion coefficient below 4.0 ppm/.degree.C., and can be fired in air at temperatures less than 950.degree. C.
Another object of the invention is to provide ceramic materials that are sintered at low temperatures to produce dense bodies (greater than 95% of theoretical density) having low coefficients of thermal expansion and a dielectric constant below 4.2 and have a glass content below 50 vol. %. A reduction in the glass content of the sintered body is very desirable in that the glassy phase is responsible for shape distortion or warpage during co-firing. If the sintered body is to be used in an electronic package, the shape distortion associated with high volume percent glass content can cause the via holes to misalign during co-firing of the metal and ceramic. A glass content below 50 vol. % will reduce the likelihood that warpage will occur.